1. Field of the Invention
The present invention relates, in general, to an arrow shaft, and more particularly to an arrow shaft having a structure divided into two parts each having different spine strengths in a longitudinal direction of the arrow shaft, so that a deformation or fracture of the center of gravity of the arrow shaft due to an archer's paradox phenomenon necessarily occurring during flight of an arrow can be inhibited, a splitting of the arrow shaft due to impact applied to the arrow shaft upon shooting can be prevented, shooting accuracy of the arrow shaft can be increased, and partial deformation or fracturing of the arrow shaft due to frequent shooting can be prevented.
2. Description of the Related Art
FIG. 1 is a schematic view showing a conventional arrow shaft according to the related art.
In general, arrows include an arrow shaft 11 having a hollow cylinder shape, an arrowhead 12 on a front end of the arrow shaft 11, a nock 13 at a rear end of the arrow shaft 11, and feathers 14 attached on a rear outer peripheral surface of the arrow shaft.
An arrow that has left the string of a bow is typically subject to a thrust which is the force by which the string pushes a rear end of the arrow. Such a thrust is transferred towards a front portion of the arrow to allow the arrow to fly.
Such an arrow undergoes an archer's paradox phenomenon when the arrow leaves and flies from the string toward a target. The archer's paradox means a phenomenon whereby an arrow flies while mainly bending from side to side in an initial flight phase immediately after the arrow has left the string.
In shooting the arrow, because kinetic energy is momentarily transferred to the arrow as if the arrow were in a stopped state, while the string is released, the arrow cannot withstand such a force. As a result, the arrow bends about a pressure point and is again straightened back to the original state by a restorative force of the arrow shaft which is an elastic body, then being bent in the opposite direction due to inertia. The arrow flies while continuously repeating such a movement until the inertial energy is dissipated.
However, for an archery arrow, the arrow is shot a few ten or hundred times a day. Therefore, the effects of such an archer's paradox on the arrow shaft can be more severe than thought. Specifically, as shown in FIG. 2, the arrow shaft can be innumerably bent about a pressure point (i.e., the center of gravity; G) in a bow shape from one direction to the others during flight. When the arrow shaft continually undergoes such a phenomenon, a middle portion of the arrow shaft where the center of gravity G is located can be deformed or fractured.
In order to overcome such a problem, there has been proposed an arrow shaft in which a hollow aluminum tube is disposed as a core inside the arrow shaft, a carbon fiber sheet is laminated on an outer surface of the aluminum tube to form a double layer, and then front and rear potions of the carbon fiber sheet are ground by a grinder to make the arrow fatter in a middle portion of the arrow shaft.
However, because the diameter of the arrow shaft of such a product is adjusted by grinding the carbon fiber sheet layer, there are the problems of maintaining the dimensions thereof, a machining defect can be readily occurred in an inside texture of the sheet layer upon grinding, and an eccentric nature can be imposed on the arrow shaft due to failure of managing the dimensions precisely. In addition, bonding between the aluminum core and the carbon fiber sheet layer which are different materials can be difficult, and the weight of the arrow shaft can be increased because the aluminum tube is included therein. This is a critical problem.
Also, because outer peripheral surfaces of the front and rear portions of the arrow shaft must be grounded to impart it with the required diameter, the waste of material can be serious and the processing time it takes to do the machining can be extended, leading to a decrease in productivity.
In particular, the carbon fiber sheet layer can peel or come off the aluminum tube due to an impact on the arrow shaft, a difference in thermal expansion coefficient between such different materials, and the like.
As described above, the archer's paradox phenomenon occurs in the arrow, as soon as the arrow is shot from the bow. In this case, if the strength, weight and length of the arrow shaft are not suitable for the strength of the bow, the arrow cannot fly in a straight line.
In general, having a stronger spin means that a strength, i.e., a spine, of an arrow is stronger than the strength of the bow, whereas having a weaker spin means that the strength of an arrow is weaker than the strength of a bow.
For this reason, in order to determine the strength of an arrow shaft, a predetermined weight is hung on the center of the arrow shaft to measure the degree of bending of the arrow shaft, and thereby the arrow shaft suited to the strength of the bow is chosen. Such a degree of bending corresponds to the above spine.
As the spine of the arrow shaft is increased, advantages include improving the capability of the arrow to fly straight and a deformation of materials due to the frequent archer's paradox phenomenon can be reduced. However, because the spine of the arrow has to be determined in consideration of the strength of the bow, it is not always advantageous for the spine to be unconditionally increased. In addition, when increasing the spine of the arrow, material and manufacturing costs can be increased.
Meanwhile, the arrow shaft is subject to different external forces depending on locations along its longitudinal direction. Specifically, the middle portion of the arrow shaft is subjected to frequently repeated bending forces due to the archer's paradox phenomenon as described above, and is likely to be weakened thereby after being used for a long term, and the front portion of the arrow shaft to which an arrow head is coupled is subject to most of the impact of hitting a target when frequently shooting. In contrast, the rear portion of the arrow shaft to which a nock is coupled is subject to most of the impact applied by a string of a bow.
Thus, the elasticity, strength and other physical properties the arrow shaft must vary depending on locations in the longitudinal direction. Therefore, although materials having different physical properties were required to be used depending on each of the locations in fabricating the arrow shaft, conventional arrows have been fabricated of only a single sheet material and such needs have not satisfied.
Therefore, in Korean Patent No. 10-1063366 to the present applicant entitled “Arrow Shaft Having Front/Middle/Rear Three-Stage Spine Structure,” the present applicant has proposed an arrow shaft which is configured by dividing and defining the arrow shaft into a front part to which an arrow head is coupled, a middle part corresponding to the center of gravity of the arrow shaft, and a rear part to which a nock is coupled, and then by laminating and winding arrow shaft shaping sheets each having different elastic strengths on each of the parts, such that spine strengths of the front, middle, and rear parts of the arrow shaft can be different from each other.
However, according to the above Korean Patent, a third sheet layer, which is an uppermost layer, is provided by bonding front, middle and rear sheets respectively formed of carbon fiber sheets each having elastic strengths which are different from each other and then by laminating and winding the resulted sheet. Therefore, there are problems that the operation is complex and the number of steps is increased, leading to decreasing the productivity.